I. Field of the Invention
The present invention relates generally to a balanced power amplifier circuit using improved power amplifier bypassing techniques. It is particularly useful in a wireless communication device, such as a CDMA wireless phone, but has other utilities as well.
II. Description of the Related Art
In various communication systems, including most hand-held wireless devices such as code-division-multiple-access (CDMA) cellular phones or any form of time-division-multiple access (TDMA) technology, RF power output from a mobile unit varies in large dynamic ranges. In a CDMA wireless system, multiple message signals are transmitted simultaneously on the same frequencies (spread spectrum). The signals are spread with different digital codes, thus allowing detection of the desired signal while the unintended signals appear as noise or interference to the receiver. Spread spectrum systems can tolerate some interference, and the interference added by each wireless transmitter increases the overall interference in each cell site. Each wireless transmitter introduces a unique level of interference, which depends on its received power level at the cell site.
The CDMA system uses power control to minimize mutual interference. Precise power control is critical to avoid excessive transmitter signal power that is responsible for contributing to the overall level of interference. The power transmitted by a particular wireless device is a function of its distance to the base station with which it is communicating and the number of other subscriber wireless devices in use talking to the same base station.
In a typical hand-held wireless unit, the power amplifier (xe2x80x9cPAxe2x80x9d) is biased class AB to reduce power consumption during periods of low transmit power, but power continues to be consumed. One arrangement to avoid continuous battery drain is to employ a switched bypass of a power amplifier, and then remove the DC power or power down the bypass amplifier. This arrangement is illustrated in FIG. 5. A single PA circuit 100 comprises a PA 102 and a circulator 104. Typically an isolator/circulator is used to isolate the PA from the effects of load impedance in subsequent stages. An RF-signal is inserted to a pole of a first switch 106. When the PA 102 is on, the first switch 106 connects the RF-input, via path 108, through a band-pass filter 110 to an input of PA 102. A second switch 114 connects the circulator 104 to ground through a terminating resistor 116. A biasing and control circuit 118 operates to bias the PA 102 and control the operation of the first and second switches 106, 114. The RF-signal is amplified and output to the circulator 104, and then transmitted to the RF-output port of the PA circuit 100.
When the PA 102 is powered down, its input and output appear as reflective impedances. Therefore the RF-signal must be routed around the PA 102. To accomplish bypassing, the first switch 106 connects the RF-input to a bypass path 112 and the second switch 114 routes the signal to the circulator 104. The RF-signal enters the circulator 104 from the second switch 114 and is routed to the output of the PA 102. The PA 102, appearing as a reflective impedance, reflects the RF-signal back to the circulator 104, which routes the signal to the RF-output port. This technique has drawbacks, however. When switches are used in the RF-output path, the switching loss must be overcome by the PA. Using the circulator 104 in the RF-output path removes the additional loss. However, using a circulator in the circuit requires another separate component from the PA, which consumes to circuit board space and cost. This can tend to cancel the benefits of bypassing. Furthermore, using a switch and a circulator requires more power to operate and is more costly to build.
FIG. 6 illustrates a prior art single PA circuit 130. An analog signal is fed from a driver amplifier 132 through a band pass filter 134 to a first switch 106. The first switch 106 alternately routes the signal between a bypass path 112 and an amplifier path 136. In the amplifier path 136, a PA 102 amplifies the signal. A circulator 104 is connected between the output of the PA 102 and the RF-output port. A second switch 114 connects the circulator to either the bypass path 112 or ground through a terminating resistor 34. A control circuit controls the first switch 106 and the second switch 114. When bypassing the PA 102, the first switch 106 and the second switch 34 route the signal through the bypass path 112. The signal then enters the circulator 104 and routes to the output of the powered-down PA 102. The signal reflects off the PA 102 and back to the circulator 104, and routes to the RF-output port. While this approach provides the benefits of bypassing, it exhibits the same drawbacks as discussed above with regard to FIG. 5, wherein the circulator burdens the circuit with the need for a separate external component from the PA package. The configuration of FIG. 5 is also costlier to build when using the separate switch and circulator components. Added costs exist when the switches are external to the PA package, as well as other components. The need for external switches from the PA package also adds to the complexity of the circuit because a purchaser of the PA package must implement the bypass switches discretely.
Between FIG. 5 and FIG. 6, the circuit of FIG. 5 is preferable because the position of the band-pass filter permits more power to get to the output port and permits the circuit to bypass the loss associated with that band-pass filter. When you are in bypass mode, more of the available power from the driver amplifier (not shown in FIG. 5) is inserted into the bypass path and losses from the band-pass filter will not be experienced The need for the band-pass filter is alleviated in the bypass mode because in this mode less power is required and out-of-band spurious emissions due to the up-converter (not shown), which are normally filtered by the band pass filter, are not large enough to be detrimental.
The advantages of the circulator method PA bypass circuit include (1) saving current by turning the PA off when the high-power levels are not needed, (2) not requiring a switch directly in the large signal path, (3) avoiding loss, (4) reducing the size and cost by removing a switch, and (5) having a stepped gain which reduces the dynamic range requirement of a variable gain amplifier which precedes the PA.
The circulator-type power amplifiers as shown in FIGS. 5 and 6 have several drawbacks, however. The circulator requires more space in the circuitry of the mobile station because it is not integrated into the PA package. The circulator method introduces undesirable variability over many of the operating conditions of the PA. Finally, the circulator method is costly to manufacture because of the extra components required.
Some benefits are experienced by replacing the circulator with hybrid circuits which split the RF-signals into in-phase and out-of-phase signals, which are independently amplified in balanced amplifier circuits. Many of the benefits of using circulators or isolators are also achieved by using balanced amplifiers. Balanced amplifiers provide a high degree of stability, are useful in broadband applications, and provide protection from load mismatch. Balanced amplifiers also ensure that the amplifier will be linear in the event of a load mismatch. Another function of the circulator which is also achieved by balanced amplifier is to provide a good 50xe2x80xa2 termination to the duplexer which follows. The duplexer is a passive filter and is designed with specific terminations. The amplifier is matched not conjugately but for maximum linear power and when you look into the output of a power amplifier, it is not 50xe2x80xa2. Thus you cannot directly connect an amplifier to a filter which is designed to work in a 50xe2x80xa2 system. Each function and advantage discussed above which is achieved by using a circulator is also achieved by using a balanced amplifier.
In a balanced amplifier, if one amplifier fails, the balanced amplifier unit will still operate but with xcx9c6 dB reduced gain. Furthermore, balanced amplifiers are easily cascaded with other units, since each unit is inherently isolated.
Circulator-type prior art single power amplifiers as discussed above and balanced power amplifiers are illustrated in U.S. Pat. No. 6,060,949, which disclosure is incorporated herein in its entirety. However, balanced amplifiers still exhibit some disadvantages for wireless telephone applications, as will be discussed below.
An example of a prior art balanced PA circuit using hybrid circuits is illustrated in FIG. 7. A balanced PA circuit 150 comprises a driver amplifier 132 producing an analog signal. The analog signal is switched by a first switch 106 between an amplifier path 136 and a bypass path 112. In the amplifier path 136, the signal is band-pass filtered 134, split by a first hybrid circuit 152 into an in-phase signal and a quadrature signal ninety degrees out of phase. The in-phase signal and the quadrature signal are each independently amplified by a first amplifier 154 and a second amplifier 156, respectively, which comprise the balanced amplifier stage. The in-phase signal is then shifted by 90xc2x0 and summed with the out of phase signal in the second hybrid circuit 158. The resulting signal is filtered by a filter (not shown) and fed toward the RF-output port.
The isolated port of the first hybrid circuit 152 is grounded by a terminating resistor 35. The bypass path 112 provides a path from the first switch 106 to an isolated port of the second hybrid circuit 158. The signal is split into an in-phase signal and a quadrature signal by hybrid circuit 158. The in-phase signal is transmitted to the output of the first PA 156, which is powered down and therefore appears as a reflective load to the signal. The quadrature signal is transmitted to the output of the second PA 154, which is also powered down and therefore appears as a reflective load to the signal. Each reflected signal enters the second hybrid circuit 158, where the in-phase signal is shifted by 90-degrees and summed with the out-of-phase signal. The summed signal is output to the RF-output port.
When bypassing the power amplifiers, a second switch 114 connects the isolated port of the second hybrid circuit 158 to the bypass path 112, and the first switch 106 routes the input RF-signal to the bypass path 112. Therefore, when bypassing, the RF-signal is input to the second hybrid circuit 158, split into an in-phase and quadrature signal, reflected off the power amplifiers 154, 156 summed again by the second hybrid circuit 158, and transmitted out the RF-output port.
When the power amplifiers are needed, the first switch 106 routes the RF-input signal to the amplifier path 136 where the signal is split by the first hybrid circuit 134, amplified by the power amplifiers 154, 156, and summed by the second hybrid circuit 158 and routed to the RF-output port. The second switch 114 connects the isolated port of the second hybrid circuit 158 to ground through terminating resistor 34, which routes any reflected signal to ground.
The PA circuit illustrated in FIG. 7 also exhibits disadvantages, however. The design cannot be fully integrated into a PA package because of the manner the switches are utilized. Furthermore, the first switch 106 causes the RF-signal to split into the amplifier path or the bypass path. Therefore, there are two RF-inputs into the PA package. These disadvantages require extra switching components outside the PA circuit in order to accomplish the benefits of bypassing. Using the balanced amplifier configuration takes up more space when compared with a single amplifier. Accordingly, effective bypassing and compact packaging not addressed by the prior art are especially important when using balanced amplifiers in CDMA wireless telephone applications.
Employing a switch directly in the large signal path is taught in the prior art. For example, U.S. Pat. No. 5,661,434 to Brozovich et al. (xe2x80x9cBrozovich et al.xe2x80x9d) discloses a multiple power level amplifier circuit. The disclosure of Brozovish et al. is incorporated herein by reference. Brozovich et al. disclose a two stage power amplifier circuit 34, 36. The first stage includes a power amplifier 26 with an input and output impedance matching networks 29, 30. The impedance matching network 30 matches the output impedance of the power amplifier 26 to the system characteristic impedance, which is defined by the load impedance at 36xe2x80x2. Thus, when the second stage amplifier 28 is bypassed, the output of the power amplifier 26 is matched.
In the low power mode of Brozovich et al., a signal-switching network in the second stage operates to bypass and power down the power amplifier 28. The switching network is controlled by a signal switch control 35, and includes an input isolation switch SW1, an output isolation switch SW2, and a bypass switch SW3. The input isolation switch SW1 is connected in series to the output of the power amplifier 28, the output isolation switch SW2 is connected in series to the output of the power amplifier 28, and the bypass switch SW3 is connected in parallel across the input isolation switch, the output isolation switch and the power amplifier 28. The extra switch SW2 at the output of the matching network from the power amplifier 28 exhibits the problems discussed above. Namely, the signal experiences insertion loss at the end of the amplifier stages due to SW2. Furthermore, switch SW2 is costly, large, and as an active device in line with the amplifier 28, must be as linear as the amplifier 28 in order to operate sufficiently. Furthermore, having active switches in series renders it difficult to integrate the switches and the amplifier in the same package.
What is needed in the art is a balanced PA circuit that is compact and which provides the benefits of bypassing the driver amplifier or balanced amplifier stage in order to conserve power.
In order to accomplish the objects of the present invention, a balanced amplifier circuit is provided which does not need an isolating switch as in Brozovich et al. or a circulator as in the prior art in order to simplify the design, space and cost of the PA circuit. The present invention decreases the cost and space required for a balanced PA circuit by removing the circulator and using couplers which can be integrated directly into the PA package. The present invention provides a PA circuit with the benefits of bypassing and which may be integrated into one PA package. Such a design provides substantially the same benefits of many of the operating conditions of the amplifier, such as load mismatch and temperature, while removing the need for an isolator or circulator. By providing a single PA package with the necessary circuit elements, a purchaser of the package need not separately implement switching using discrete components to accomplish bypassing. Furthermore, not requiring a switch in the large signal path avoid the loss at the end of the switch as well as the cost and size of that switch which are needed to avoid or reduce the loss and hence increased power consumption. An aspect of the present invention it to employ a bypassing method which avoids the need for a switch in the high power path.
These advantages and others may be realized by the invention disclosed herein. According to the first embodiment of the present invention, in a PA circuit, a band-pass filtered single RF-input signal is provided to a normal balanced amplifier. The amplified signal is input to a first coupler. The first coupler splits the signal and transmits an in-phase signal and an out-of-phase signal to a first PA and second PA, respectively. The first and second power amplifiers constitute the balanced stages of the circuit. A first amplified signal and a second amplified signal are respectively produced. The balanced amplifier stage transmits the first and second amplified signals to a second coupler. The first coupler""s isolated port is connected via a first switch to a terminating resistor. Likewise, the second coupler""s isolated port is connected via a second switch to a terminating resistor. A pole of the first switch and a pole of the second switch connect to a bypass path that connects the isolated ports of the first and second couplers together. The second coupler transmits a combination of the in-phase and out-of-phase amplified signals to the RF-output port.
In one mode of operation, the PA operates as a normal balanced amplifier, with the isolated ports of the respective first and second couplers terminated via the first and second switch respectively, to terminating resistors. In a second mode of operation, the balanced stages are powered down and thus present a highly reflective impedance at both output and input of the amplifier. In the second mode of operation, both the first and the second isolated ports of the couplers are connected together via the first and the second switches and the bypass path.
In the second mode of operation, the first coupler splits the input RF-signal into an in-phase signal and an out-of-phase signal between the balanced stages. The first and second PA, which are powered down and appear as reflective impedances, reflect the split signals. The reflected signals combine at the isolated port of the first coupler. The signal passes through the bypass path to the isolated port of the second coupler, which splits the signal into an in-phase signal and a out-of-phase signal. The in-phase and out-of-phase signals reflect off the output of the power amplifiers in the balanced stage. The second coupler combines the reflected signals and routes the combined signal to the RF-output port. In the preferred embodiment, the first switch is integrated into the PA package. But both switches may be integrated as well. This embodiment provides the advantage of using passive components in the couplers and using their isolated ports for bypassing which is achieved in a single integrated PA package without introducing active components.
According to a second embodiment of the present invention, the second isolated port of the second coupler of the first embodiment is connected directly to a terminating resistor without a switch. A third switch switches the RF-input between the PA path and a bypass path that bypasses the driver stage 1 and is connected to the first switch for inserting the RF-signal into the first coupler. In a first mode, the PA operates as an ordinary balanced amplifier, with high power output capability and gain. In a second mode, the driver stages are shut down and the RF-signal is input into the first coupler via the first isolated port. In this embodiment, the driver stages are bypassed and the coupler, driver stages, and the first switch are all integrated into a PA package. In the preferred implementation of this embodiment, the band pass filter is not integrated into the PA package but it is conceivable that it may be integrated into the package.
According to a third embodiment of the present invention, the single RF-signal is input to the driver amplifier stage, and the amplified signal is input to the first coupler. The isolated port of the first coupler is grounded. The first coupler routes the in-phase and out-of-phase signals to the first and second power amplifiers in the balanced amplifier stage. The amplified signals are input to the second coupler. The isolated port of the second coupler is connected via the second switch to either the 50 xcexa9 termination or the output of a signal processor. The signal processor transmits a signal that cancels distortion caused by the power amplifiers. The signal processor generates the distortion compensation signal by coupling the input and output signals to a signal processing system, which may be either digital or analog or a mixture of the two. This minimizes the distortion in the RF-output signal. Since the injected signal is derived from the output signal, and reflects off the amplifier stages in the balanced amplifier, the distortion compensation signal changes the driving impedance seen by those devices.
In another aspect of the present invention, in each case discussed above where the isolated port of a coupler is terminated, the coupler may be replaced with any network that splits the power into equal amplitudes and introduces a quadrature 90-degree phase shift. The substitute network may be active or passive, and contain lumped and/or distributed passive elements. The advantages of the present invention generally involve using the isolated ports of the passive couplers to enable bypassing in a balanced amplifier configuration and to enable a single PA package to include the necessary integrated elements to achieve bypassing which reducing the need for discrete switches or other active components external to the package. It is also conceivable to use couplers or other equivalent networks known to those of ordinary skill in the art which introduce a 180-degree phase shift.